如何在后台服务颤振中调用mqtt客户端

我的项目使用mqtt进行通知。需要实时通知。 我有这样的mqtt类

class Mqtt {
...

  Mqtt._() {
    mqttClient = MqttServerClient.withPort(server, clientIdentifier, port);

    ///Successful connection callback
    mqttClient.onConnected = _onConnected;

    ///Disconnected callback
    mqttClient.onDisconnected = _onDisconnected;

    ///Subscription success callback
    mqttClient.onSubscribed = _onSubscribed;

    ///Subscription failure callback
    mqttClient.onSubscribeFail = _onSubscribeFail;
  }

  static Mqtt getInstance() {
    if (_instance == null) {
      _instance = Mqtt._();
    }
    return _instance!;
  }

  ///connection
  connect() {
    mqttClient.connect();
    _log("connecting");
  }

...
}

我这样打电话

Mqtt mqtt = Mqtt.getInstance();
mqtt.subTopic = 'NOTIFICATION/';
mqtt.connect();

但当应用程序停止时,mqtt会断开连接。 如何在后台服务颤振中调用mqtt客户端?